The invention relates to a television camera tube comprising, in an evacuated envelope, a diode electron gun for generating an electron beam. The electron gun comprises, an anode, and a focusing lens. The cathode has a cathode having an emissive surface extending substantially perpendicular to the axis. The anode has a central aperture around the axis. The focusing lens focuses the electron beam on a photosensitive target. A potential distribution is formed on the target by projecting an optical image on it. The target provides electrical signals corresponding to the optical image by scanning with the electron beam.
Such a television camera tube is disclosed in U.S. Pat. No. 3,831,058 (Van Roosmalen). The television camera tube described in that patent comprises a diode electron gun in which, during scanning, the current density of the electron beam at any point along the axis between the cathode and the anode is at most three times the current density at the point of intersection of the axis with the cathode. In order to reduce the beam current inertia, it is important to restrict the number of interactions between the electrons of the electron beam.
However, diode electron guns have the disadvantage of producing a considerable anode current. Since the cathode emits over a very large part of the emissive surface, and since the emissive surface of the cathode is in practice must larger than the area of the aperture in the anode, a very large part of the electron beam current in a diode electron gun is intercepted by the anode. The part of the electron beam current which is intercepted by the anode is the anode current. The anode current causes extra power dissipation, in particular when dynamic beam current control is used. Restricting the emissive surface by making the cathode smaller is not attractive because as a result of this the lifetime of the cathode and hence of the camera tube is reduced.
In Netherlands Patent Application No. 8002037 (published as U.K. Patent Application No. 2,057,750), a television camera tube is described having a diode electron gun in which the anode current is restricted. The anode used in this diode electron gun is funnel-shaped, so that the part of the anode which forms the aperture is situated nearer the cathode than the remainder of the anode. This funnel-shaped part has an area which is less than 75% of the emissive surface of the cathode. As a result of this shape, the anode current is reduced.